Mosaic x-ray pick-up screen for x-ray image intensifier tubes



Oct. 28, 1969 ESHCER I 3,475,411

MOSAIC X-RAY PICK-UP SCREEN FOR X-RAY IMAGE INTENSIFIER TUBES men VOLTAGE comma Filed Def. 27, 1966 X-RAY GENERATOR l3 VIEWING-M SCREEN I FIG. I

PRIOR ART PRIOR ART FIGQ4 I 5I "Q 1""5 I "INVENTORY WILLIAM E. SPICER TTORNEY United States Patent U.S. Cl. 250213 8 Claims ABSTRACT OF THE DISCLOSURE The present invention relates in general to X-ray age intensifier tubes and, more particularly, to an 1mproved intensifier tube employing a curved X-ray pick-up screen formed by a mosaic of flat pieces of alkali metal halide fluorescent material, whereby a generally spherical pick-up screen is formed of such improved material without encountering distortion of the picked up X-ray image, as would otherwise be obtained if a flat slab of such material were to be formed into the spherical shape. Such improved X-ray image intensifier tubes are especially useful in X-ray systems and for intensifying gamma ray images in nuclear medicine applications.

Heretofore, X-ray image intensifier tubes have been built which employ a generally spherical X-ray pick-up screen made of an X-ray sensitive fluorescent material such as ZnS. The ZnS screen is typically made by settling particles of ZnS out of a liquid slurry onto the inside surface of a spherical X-ray. transparent pick-up face plate of the image intensifier tube. The particulated ZnS has only about half the density of bulk ZnS.

higher intrinsic absorption for X-rays. Such improved I materials include the fluorescent alkali metal 'halides such as, for example,.CsI, RbI, NaI, KI etc. Such materials are commercially available in flat slab .form. However, due to the ionic nature of these materials, when the flat slab material is deformed to conform to the desired spherical shape of the pick-up face plate, the conversion efiiciency of the alkali halide material is greatly reduced due to the plastic deformation of the material.

In the present invention, the spherical X-ray sensitive phosphor screen is made by a mosaic of separate flat pieces of alkali metal halide phosphor overlaying the spherical pick-up face. While the individual segments of the mosaic pick-up screen are fiat, the overall shape is generally spherical. In this manner, the advantages 'of the improved X-ray sensitive phosphor material are obtained in a curved pick up screen geometry Without substantial loss of conversion efiiciency otherwise encountered due to distortion of the slab material.

The principal object of the present invention is the provision of an improved X-ray image intensifier tube.

One feature of the present invention is the provision of a curved X-ray pick-up screen made of a mosaic of fiat pieces of alkali metal halide phosphor, whereby the advantages of such improved material are obtained without substantial loss of conversion efllciency and thus loss of resolution in the converted X-ray image.

ice

Another feature of the present invention is thesam'e as the preceding feature wherein the flat s'egments 'forming' the mosaic are rectangular, whereby theseparate pieces of the mosaic have side edges parallel to crystallographic planes of the material.

Another feature of the present invention is the-provision of a conductive coating of material provided around the side edges of the mosaic segments, whereby an electrical conductive path is provided through the phosphor mosaic to the photo-cathode which overlays the mosaic. *1

Another feature of the present invention is the rovision of a mosaic of fiat surfaces formed on the interior surface of the spherical X-ray transparent face plate of the image intensifier tube to receive the fiat segments of the mosaic X-ray pick-up screen to provide supporting contact for the individual mosaic segments over substantially their entire facial area abutting the faceplate, whereby improved resolution of the pick-up image'is ob; tained. i

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic line diagram of an X-ray image intensifier tube of the prior art,

FIG. 2 is an enlarged cross-sectional view of a portion of the structure of FIG. 1 delineated by line 22,

FIG. 3 is a view similar to that of FIG. 2 depicting the pick-up screen construction of the present invention,

FIG. 4 is an enlarged view of the structure of FIG. 1 taken along line 4-4 in the direction of the arrows and depicting the mosaic pick-up screen of the present invention,

FIG. 5 is an enlarged sectional view of the structure of FIG. 4 taken along line 5--5 in the direction of the arrows, and

FIG. 6 is an enlarged detail of an alternative structure to that of FIG. 3.

Referring now to FIG. 1 and to FIG. 2 there is shown a prior art X-ray system employing an X-ray image intensifier tube 2. Such a system is described in an article entitled, X-Ray Image Intensification With a Large Diameter Image Intensifier Tube, appearing in the American Journal of Roentgenology Radium Therapy and Nuclear Medicine, vol. 85, pp. 323-341 of February 1961. Briefly, an X-ray generator 3 serves to produce and direct a beam of X-rays onto an object 4 to be X- rayed. The image intensifier tube 2 is disposed to receive the X-ray image of the object 4. i i

1 The image intensifier tube 2 includes a dielectric vacuum envelope 5 as of glass approximately 17 incheslo'ng' and 10 inches in diameter. The pick-up face portion6 of the tube 2 comprises a spherical X-ray transparent portion of the envelope 5, as of aluminum or conductive gla ss, which is operated at cathode potential. An image pick-hp screen 7 made of X-ray sensitive particulated phosphor such as ZnS is coated onto the inside spherical surface'of the faceenvelope face portion 6 to a thickness as of 0.020". A chemically inert optically transparent buffer layer g is coated over the phosphor layer 7. A photo-cathode layer 3 is formed over the buffer layer 8. 1 I In operation, the X-rays penetrate theobject '4 to be observed. The local X-ray attenuation depends on i th' the thickness and atomic number of the e'lernentsfo In; ing the object under observation. Thus, the intensity pat tern in the X-ray beam after penetration of the object4 contains information concerning the structure of theob-i ject. The X-ray image passes through the envelope fa ce portion 6 and falls upon the X-ray sensitive phosphor layer 7 wherein the X-ray photons are absorbed and re emitted as optical photons, typically in the blue frequency ilange. The optical photons pass through the transparent buffer 8 to the photo-cathode 9 wherein they produce electrons. The electrons are emitted from the photocathode in a pattern or image corresponding to the original X-ray image. The electrons are accelerated to a high velocity, as of 30 kv., within the tube 2 and are focused through an anode structure 12 onto a fluorescent screen 13 for viewing by the eye or other suitable optical pick-up device. Electron focusing electrodes 14 are deposited on the interior surfaces of the tube 2 to focus the electrons through the anode 12.

In the intensifier tube, one 50 kev. photon of X-ray energy absorbed by the X-ray sensitive pick-up screen produces about 2,000 photons of blue light. These 2,000 photons of blue light produce about 400 electrons when absorbed in the photo-cathode layer 9. The 400 electrons emitted from the photo-cathode produce about 400,000 photons of light in the visible band when absorbed by the fluorescent viewing screen 13. Thus, the X-ray image is converted to the visible range and greatly intensified for viewing.

l One of the problems with the prior art intensifier tube 2 is that the particulated pick-up screen has less than optimum resolution due to the fact that the particulated material has about one-half the density of the material in bulk form. Thus, to provide a certain probability of stopping or absorbing an X-ray photon, the particulated layer 7 must have about twice the thickness of such a layer if it had bulk density. The thicker the layer 7 the poorer its X-ray resolution. Moreover, the particulated material serves to scatter the emitted optical photons, thereby still further reducing resolution.

In addition, it is desirable to utilize a pick-up screen material having a greater intrinsic stopping or absorbing power ,for X-rays. Such improved materials include the alkali metal halides such as, for example, CsI, KI, NaI, RbI, CsBr and LiI. These improved materials such as CsI and NaI are obtainable in bulk slab form from Harshaw Chemical Company of Cleveland, Ohio. However, when the slabs are distorted from the flat slab form into the spherical slab form, to conform to the spherical pick-up face 6 of the image intensifier tube 2, it is expected that the efiiciency and resolution of the converted X-ray image is deleteriously affected due to plastic deformation of the material.

Referring now to FIG. 3 there is shown a section of the X-ray pick-up screen formed in accordance with the present invention. More particularly, the alkali metal halide pick-up screen layer 16 is formed on the spherical X-ray transparent substrate member 6 by adhering a multitude of rectangular slab segments in the form of a mosaic to the spherical face plate 6. The buffer and photo-cathode layers 8 and 9 are formed over the mosaic screen layer 16 in the conventional manner.

Referring now to FIGS. 4 and the mosaic screen 16 is shown in greater detail. The individual slab segments 17 are formed by slicing up a thin slab of X-ray sensitive fluorescent material selected from the class consisting of the alkali metal halides and including CsI, RbI, NaI, KI, LiI and CsBr. Such material is available in either bulk or slab form from Harshaw Chemical Company of Cleveland, Ohio. The bulk material is preferably sliced into thin slabs as of 0.010" to 0.100 thick and in diameter. The thin slab is then preferably sliced as by a wet string saw into squares 17 about /2 cm. on a side. The flat slab segments 17 are then aflixed as by silver chloride flux to the spherical face plate 6. In this step, both the receiving surface of the face plate 6 and the back and side surfaces of the screen segments 17 are coated with silver chloride flux. As an alternative, these same surfaces may be coated with a low vapor pressure epoxy cement such as that marketed by Varian Associates under the name of Torr Seal.

The flat slab segments 17 are then fitted into place on the spherical face of the face plate 6 to form the mosaic an oven and heated to the melting point of the silver chloride. The silver chloride melts and wets both abutting surfaces to provide a good bond therebetween. If the epoxy is used as the adhesive, the mosaic screen need not be heated to harden the epoxy cement.

In a preferred embodiment of the present invention, the photo-cathode 9 is preferably formed directly on the mosaic screen 16 without the provision of the intermediate buffer layer 8, as shown in FIG. 6. An X-ray image intensifier tube employing the photo-cathode formed directly on the pick-up screen is described and claimed in copending U.S. application 606,513, filed Dec. 27, 1966, and assigned to the same assignee as the present invention.

In the case where the photo-cathode 9 is formed directly onto the screen layer 16, a conductive path from the conductive face plate 6 to the photo-cathode should be provided in order to provide an electrical path for replenishing the electrons drawn from the photo-cathode by photo-emission. Accordingly, the slab segments 17 are preferably coated on their adjacent side edges and back face portions abutting the face plate 6, with a conductive material such as platinum paint. The paint comprises particles of platinum in an organic binder. Such paint is available from Englehart Industries. The painted slab segments are then baked to evaporate the organic binder leaving the conductive platinum coating on the painted surfaces.

The platinum coated surfaces are then coated with the silver chloride flux, as aforedescribed, and aflixed to the face plate 6 and to each other as aforedescribed. The platinum coating provides a conductive current return path 18 from the face plate 6 to the photo-cathode layer 9 which is formed on the screen 16.

In a preferred embodiment of the present invention, the curved face of the spherical face plate 6, which receives the slab segments 17, is formed with a multitude of flat facets 19 of the same dimensions as those of the rectangular slab segments 17. In this manner, the fiat slab segments 17 are supported from flat faces on the curved face plate 6, thereby obtaining improved support for the slab segments 17 and preventing unwanted plastic deformation thereof.

As used herein, X-ray is defined to mean photons of an energy level equal to those of X-rays or higher. Thus, X-ray as used herein means not only X-rays but gamma rays and other high energy photons.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention can be made without departing from the scope thereof it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An X-ray image intensifier tube of the type having an evacuated envelope structure with a curved X-ray transparent pick-up face portion, a layer of X-ray sensitive phosphor formed on and curving with the face portion to provide an X-ray pick-up screen, a photo-cathode layer overlaying said pick-up screen to convert the optical images produced by the X-rays in the pick-up screen into electron images, an electron accelerating and focusing structure for accelerating the electron images to relatively high energy and means for picking up the high energy electron images for viewing, the improvement comprising, said X-ray pick-up screen being a mosaic of fiat slab segments of a fluorescent alkali metal halide material, whereby a curved screen geometry of such material is obtained without undue distortion of the intensified X-ray image.

2. The apparatus of claim 1 wherein said flat slab segments making up the mosaic are rectangular, whereby the edges of the slabs can parallel a crystallographic plane of the crystalline material.

3. The apparatus of claim 1 including a conductive coating provided on the side edges of said flat slab segments of said mosaic, and wherein said photo-cathode layer is formed directly on said mosaic pick-up screen, whereby said conductive side edges provide a conductive path through said pick-up screen to said photo-cathode to replace photo-current drawn from said photo-cathode.

4. The apparatus of claim 1 wherein said curved face plate portion which abuts said mosaic pick-up screen is formed with a mosaic or flat facets to receive said flat slab segments of said mosaic pick-up screen, whereby fiat support surfaces are provided for supporting said flat segments of said pick-up screen.

5. The apparatus of claim 1 wherein said pick-up screen material is selected from the class consisting of CsI, RbI, -NaI, KI and Lil.

6. The apparatus of claim 1 wherein said flat slab segments of said mosaic are from 0.010" to 0 .100" thick.

7. The apparatus of claim 1 including an adhesive selected from the class of silver chloride and epoxy resin bonding said mosaic segments to said face plate.

8. The apparatus of claim 3 wherein said conductive coating is platinum.

References Cited UNITED STATES PATENTS OTHER REFERENCES Gray, D. E., American Institute of Physics Handbook, 1963, McGraw-Hill, pp. 9-136 to 9-137 relied upon.

WALTER STOLWEIN, Primary Examiner US Cl. X.R. 313-66 

